The disclosure relates to a method for operating a braking device, in particular a parking brake device, which comprises an actuator having an electric motor that displaces an actuator element as desired into a brake application position or into a brake release position, wherein the electric motor is controlled in dependence upon a motor constant and an electrical resistance of the electric motor.
Moreover, the disclosure relates to a device for operating a braking device of this type having the features of the disclosure and a braking device having the features of the disclosure.
Methods and apparatuses and also braking devices of the type mentioned in the introduction are known from the prior art. Braking devices having an electrically motorized actuator are known by way of example from DE 197 32 168 C1. In order to operate the braking device, it is of importance that a defined brake application force is achieved with which the braking device is actuated. The term ‘brake application force’ is understood to mean the clamping force that is set between brake linings, which are influenced by a force by means of the actuator element, and a brake disc. Usually, a rotary movement of the electric motor is converted for this purpose by way of a gear spindle unit into a translatory movement of the actuator element. It is necessary during each procedure of controlling the electric motor so as to initiate a brake application procedure to first take up two no-load travel distances, namely the play that is always present at the start between the actuator element and possibly the brake piston that is to be displaced and pressed against the brake disc by means of the actuator element, and the play between the brake linings and the brake disc. A normal force, the so-called brake application force, is subsequently built up on the brake disc. This brake application force must achieve a defined minimum value under all specified operating conditions. However, for reasons of cost, the force is not measured directly.
Furthermore, it must be ensured for a brake release procedure that the actuator element is moved within a predetermined travel distance. It is necessary to ensure that also from the point of view of brake lining wear that after the completion of the brake release procedure no residual braking torque remains as a result of the travel distance being too short. Moreover, the distance travelled must not be too large, which—without a further calibrating measure—could result in an increased closing time period during the next subsequent brake application procedure. In order to determine the brake application force and the travel distance of the actuator element as accurately as possible, it is therefore known to ascertain typical motor parameters and in dependence thereon to control the electric motor. Typical motor parameters are in particular a motor constant of the electric motor and also the electrical resistance of the electric motor. With the knowledge of the motor parameters, in particular the motor constant, it is possible during a brake application procedure to determine or estimate the output torque of the electric motor (torque) and thus also the clamping force or brake application force in a sufficiently accurate manner. During the brake release procedure, in other words as the actuator element is displaced into its brake release position, it is possible with the knowledge of this information to accurately calculate the travel distance.